1. Field of the Invention
This invention concerns a part-reflecting mirror device for use in an optical instrument and, although the invention is not necessarily so restricted, it is more particularly concerned with a measuring instrument, such as a plotter, provided with such a mirror device, for producing a plot, contour or measurement of a small object of complicated shape, e.g. a pot shard or a denture cast used in the study of orthodontics.
2. Prior Art
Any form of direct measurement of such an object is necessarily limited, and individual measurements of parts of such an object usually have to be effected independently of each other since a measuring tool capable of reaching all points of such an object is very difficult to design. Detailed measurement of such an object in a defined and constant reference system is usually practical only in some form of indirect measurement where the necessary instrument is not in contact with the object. For example, the object may be photographed from two positions, and the two photographs may be used in a stereo-plotting instrument to provide a plot of the object from which the required measurement or measurements may be made. Such stereo-plotting instruments, however, are very expensive, necessitate considerable operator training, and are subject to the errors arising from photographic distortion.
If, however, an attempt were to be made to produce the required plot of the object by disposing the latter on one side of a conventional part-reflecting mirror device and producing the plot by moving a measuring mark or index about an image of the object on the opposite side of the conventional mirror device, parallax errors would occur by reason of the fact that some of the rays of light which would be used to produce the plot would pass through the mirror device and be refracted thereby, while other rays of light would merely be reflected by the mirror device without passing therethrough, and there would thus be differences in the effective optical paths of the rays.